1. Field of the Invention
The present invention relates to a data transfer system using mobile IP. In particular, the present invention relates to a data transfer system using mobile IP that makes possible efficient seamless data transfer for a mobile terminal moving between foreign networks.
2. Description of the Related Art
In the Internet, a network address portion of an IP address assigned to a mobile host (MH) becomes an address peculiar to a home network (HN) to which the MH belongs. When the MH has moved to a foreign network(FN), it is necessary to newly acquire an IP address containing an address peculiar to the FN.
On the other hand, as a technique for continuing connection without changing an IP address as if the MH is connected to the HN even in the case where the MH is moved from the HN to the FN, the mobile IP prescribed in RFC (Request For Comments) 2002 has been standardized.
In order to implement the mobile IP, a node called home agent (HA) is disposed in the HN and a node called foreign agent (FA) is disposed in the FN as shown in FIGS. 9 to 12.
Since the MH has already acquired an IP address that can be utilized in the HN, the MH can directly receive IP packets while the MH is connected to the HN. When the MH has moved to an FN, or when the MH has moved between FNs, the MH recognizes the FN to which the MH itself belongs, by receiving an agent advertisement (AA) message publicized periodically by an FA of movement destination. As shown in FIG. 9, the MH places the IP address of the FA of the movement destination on an RRQ (registration request) message, and notifies it to the HA of the HN. In response to the RRQ message, the HA transfers an RRP (registration reply) message to the MH via the FA and completes the location registration.
Thereafter, all IP packets sent from an opposite terminal CN toward the MH are taken over by the HA as shown in FIG. 10. The HA encapsulates (IP-tunnels) IP packets having the IP address of the MH as destination by using IP packets having the IP address of the FA as destination, and transfers resultant IP packets to the FA via the Internet. From the encapsulated IP packets, the FA restores the original IP packets directed to the MH. The FA transfers the restored original IP packets to the MH via an access point AP.
As for data directed from the MH to the CN, the data are transferred to a GWR whose address has been acquired by an AA message transmitted from the FN as shown in FIG. 11. As a result, the data are transferred to the CN according to ordinary IP routing.
If the MH moves from FN1 to FN2 as shown in FIG. 12, then the MH establishes an association with an AP of movement destination, thereafter issues a registration request to the HA, and causes the HA to transfer data directed to the MH.
In the conventional communication using mobile IP, the intervals at which the FA sends AA messages are at least approximately once per second. As a matter of fact, the lifetime of AA messages is defined as at least three (3) seconds. During at least this period, the MH does not send the RRQ message. Therefore, the time required for the HA to recognize movement of the MH becomes long. Until the location registration is completed, all data directed to the MH are transferred from the CN to the FN1, which is the FN before the movement. In other words, transfer interruption is caused until location registration at the movement destination is completed. The MH attempts to transfer data directed to the CN to the GWR of the FN1 preceding the movement. However, the data are not transferred because the GWR of the FN1 is in another network. In this case as well, transfer interruption is caused.